1. Field of the Invention
This invention relates to pressure measurement apparatus of the triode ionization gauge type which is widely employed for measuring the pressure due to residual gases in a partially evacuated vessel.
2. Description of the Prior Art
An ionization gauge consists of a gauge tube and a controller therefor. One standard triode ionization gauge tube consists of a central wire collector electrode forming the axis of a surrounding cylindrical open cage anode electrode which is maintained at a positive potential with respect to the collector. A hot-wire filament is ordinarily disposed outside of the cylindrical structure parallel to the axis thereof and maintained at a potential intermediate the grid and collector potentials. In operation, electrons emitted from the filament are accelerated toward the higher potential anode. The accelerated electrons acquire sufficient energy to ionize the residual gas and the resulting positive ions, formed at a positive potential within the cylindrical anode structure, are in turn attracted to the collector electrode at ground potential causing a positive ion current, I.sub.i, to flow (through measuring apparatus) to ground. The electrons injected into the interior of the cylindrical region are ultimately collected on the anode structure causing an electron emission current, I.sub.e, to flow in in the external filament-anode circuit.
Pressure measurement is realized with this apparatus because the ion current, which is readily measured, is proportional to the number of neutral atoms present (the pressure) and the number of electrons available to initiate the ionization process. The latter is proportional to the current flowing in the external anode-filament circuit. Expressed compactly
P = k (I.sub.i / I.sub.e ) (Eq. 1)
where P is the pressure, I.sub.i and I.sub.e are as defined above and k is a proportionality constant. In the conventional prior art controller, the emission current is maintained at a constant value. Therefore, the above expression simplifies to simple linear relationship
P = k' I.sub.i (Eq. 2)
and pressure is obtained directly from a simple mesurement of the positive ion current and I.sub.e has become a fixed value and absorbed into the proportionality constant.
Close regulation of the emission current is required in such prior art controllers to compensate for variation in emission current which may be occasioned by a variety of factors, e.g., line voltage fluctuations or effects due to various ambient gases and the surface contamination of the emitting surfaces. Therefore, in the absence of emission current regulation, these variations in emission current would limit the degree of stability of the quantity represented as the denominator in Equation 1 with resulting lack of precision of the pressure measurement. This error is exacerbated in those ranges of very low pressure where the absolute value of emission current is substantially larger than the ion current and the magnitude of the corresponding fluctuations in emission current may approach and perhaps exceed the magnitude of the ion current.
It is also apparent that emission current must be increased with decreasing pressure, in order to provide a measurable ion current at low pressure because a smaller probability of ionization due to fewer residual gas molecules must be balanced by a greater number of ionizing electrons. The conventional prior art controller circuit requires the stabilization of the emission current at a number of different values in order to maintain a calibrated response over a desired pressure range. It is thus necessary, with such prior art controllers, to divide the broad dynamic range of such an instrument into several pre-selected sub-ranges, usually chosen for convenience to correspond to decades of the measured quantity. For each sub-range or decade, the emission current is stabilized at a corresponding value required to give measurable ion current.
Such prior art controller circuits undertake to protect the gauge tube from exposure to overpressure conditions by various means. Usually this protection takes the form of de-energizing a relay to turn off the emission current to the gauge tube filament when the pressure exceeds a predetermined level. The sensing of this level is ordinarily accomplished independently by a separate gauge, e.g., a thermocouple gauge or the like. Practical problems attend sensing the protection level with the pressure proportional output of the ionization gauge itself although this approach is sometimes attempted. The problem is that sensing pressure as a function of the collector ion current is deceptive at higher pressure because there is a double valued response of ion current to pressure. In other words, ion current goes down with decreasing pressure at low pressures and goes down with increasing pressure at high pressures. In the limit of high pressure, for example above about 100 microns, recombination processes become prevalent and the ions produced within the grid structure are subject to these processes with a probability which increases with increasing pressure. More specifically, gas atoms which become ionized and should be counted as ion current are instead neutralized by recombination with electrons and thus do not reach the collector. As a result, the ion current is a decreasing function of pressure in this pressure region. Thus at high pressure, the conventional arrangement indicates that the pressure is decreasing and there is no apparent need to turn the gauge off.
In addition to such conventional controller circuits which have been widely used in industry, proposals have been made to measure the ratio of ion current over emission current in order to give a pressure measurement which would not require the conventional precise stabilization of emission current at a fixed constant value. However, these proposals have involved some type of control over the emission current, and they do not provide any means to indicate overpressure in a manner which avoids the inaccuracy caused by the recombination phenomenon at high pressures.